The present invention relates to a magnetic resonance imaging (MRI) system for obtaining image information reflecting information of spin density and/or relaxation time of specific atomic nuclei present in an object to be examined using a magnetic resonance (MR) phenomenon and, more particularly, to a medical diagnosis MRI system for a human body, i.e., a patient as an object to be examined.
In an MRI system, a tomographic image in a selected slice of a patient is obtained as follows.
As shown in FIG. 1, very uniform static magnetic field H0 is applied to patient P. Thus field H0 extends in a z direction. Any atomic nucleus resonates in static magnetic field H0 at angular frequency W0 represented by the following equation: EQU .omega.0=.gamma..H0 (1)
In equation (1), .gamma. is a gyromagnetic ratio. Gyromagnetic ratio .gamma. is unique to each type of atomic nucleus, and differs in accordance with the types of atomic nuclei.
A magnetic field gradient along the z axis, i.e., linear gradient magnetic field Gz having a magnetic field intensity distribution linearly proportional to displacement in the z direction, is superposed on static magnetic field H0 by a pair of gradient coils 1A and 1B. (Linear gradient magnetic field Gz also has the direction of magnetic field, i.e., the direction of field line along the z direction.) Linear gradient magnetic field Gz has magnetic field intensities which are different for each displacement along the z axis. Thus, for example, an x-y plane portion in FIG. 1, i.e., specific slice portion S (although it looks like a planar portion, it has a certain thickness in practice) has a predetermined magnetic field intensity due to, e.g., only static magnetic field H0 if the field intensity due to gradient field Gz at slice portion S is zero (the field intensity due to gradient field Gz gradually increases on one side of slice portion S and gradually decreases on the other side of slice portion S, in accordance with displacement in Z-direction). More specifically, a slice for obtaining a tomographic image can be selectively determined by linear gradient magnetic field Gz.
Rotating magnetic field H1 at angular frequency .omega.0 for resonating only specific nuclei is applied to patient P to be superposed on static magnetic field H0 and gradient magnetic field Gz through a pair of transmission coils 2A and 2B arranged on a probe head. In this way, rotating magnetic field H1 acts on only on slice portion S which is selectively determined by linear gradient magnetic field Gz, and the MR phenomenon is caused in slice portion S.
The MR phenomenon is observed as an MR signal through a pair of receiving coils 3A and 3B arranged on the probe head. The observed MR signal is Fourier transformed, thereby obtaining a unique spectrum for the angular frequency of the specific atomic nucleus spin.
In order to obtain a tomographic image by image reconstruction, an MR signal including positional information for x and y directions in the x-y plane of slice portion S must be generated. For this purpose, frequency and phase information is used as media of positional information in the MR signal.
As shown in FIG. 2A, after slice portion S is excited to cause the MR phenomenon, a magnetic field gradient along the y axis, i.e., linear gradient magnetic field Gy having a magnetic field intensity distribution linearly proportional to displacement in the y direction, is superposed on static magnetic field H0. (Linear gradient magnetic field Gy also has the direction of magnetic field, i.e., the direction of field line along the z direction.) In the MR phenomenon, phase difference .phi.y, represented by the following equation, due to linear gradient magnetic field Gy is caused for the displacement in the y direction: EQU .phi.y=.gamma..multidot.Gy.multidot.y.multidot..pi.=.omega.y.multidot..pi.( 2)
Causing a phase difference upon application of a gradient magnetic field, in this manner, is called "phase encoding".
When an MR signal is detected while superposing a gradient magnetic field along the x axis, i.e., linear gradient magnetic field Gx having a magnetic field intensity linearly proportional to displacement in the x direction on static magnetic field H0 (linear gradient magnetic field Gx also has the direction of magnetic field, i.e., the direction of field line along the z direction), the MR signal causes linear frequency difference .omega.x represented by the following equation for the displacement in the x direction: EQU .omega.x=.gamma..multidot.Gx.multidot.x (3)
MR excitation and MR signal acquisition as above are repeated n times. Upon repetition, phase encoding linear gradient magnetic field Gy is changed for each cycle, and .phi.y given by equation (2) is varied for each cycle. For example, signals corresponding to positions A, B, and C in FIG. 2A are obtained in accordance with the first gradient magnetic field Gy, as shown in the upper left part of FIG. 2B, signals corresponding to positions D, E, and F in FIG. 2A are obtained in accordance with the second Gy, as shown in the left middle part of FIG. 2B, and signals corresponding to positions G, H, I in FIG. 2A are obtained in accordance with the third Gy, as shown in the lower left part of FIG. 2B, thereby obtaining signal F (t,n) (the right part of (FIG. 2B) represented by the following equation: ##EQU1## Note that .rho.(.omega.x,.omega.y) in equation (4) is the frequency spectrum of signal F(t,n). When signal F(t,n) is two-dimensionally Fourier transformed, .rho.(.omega.x,.omega.y) is obtained, and a tomographic image in slice portion S can be reconstructed. Such an imaging operation is called a "two-dimensional Fourier transformation (2DFT) method".
However, the MRI system employing the 2DFT method requires several minutes or more for acquiring MR data necessary for image reconstruction. For this reason, a patient cannot hardly stand still while MR data necessary for image reconstruction is acquired. In the MRI system employing the 2DFT method, artifacts caused by the body movement of the patient due to, e.g., his respiration, during MR data acquisition are formed on the resultant MR image. The artifacts caused by the body movement are so-called ghost-like artifacts occurring in a direction corresponding to the phase encoding direction on the MR image.
Contrary to this, a high-speed imaging method in which image reconstruction is performed using MR data obtained in a very short period has been developed. Such a high-speed imaging method cannot obtain an MR image having a good quality, e.g., good contrast, compared to a low-speed imaging method such as the 2DFT method.